1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image by an electrophotographic method, and more specifically, it relates to an image forming apparatus, for example, a photocopier, a printer, a facsimile, or a multifunctional machine.
2. Description of the Related Art
The steps involved in forming a color image by the color electrophotographic method will be described. A color image to be formed is separated into different images with a red, green, and blue filter. The surface of an image carrier is charged and is then exposed to light to form an electrostatic latent image on the surface of the image carrier. A toner image is then formed with a toner and is then transferred to a transferring member. This process is repeated to form a color toner image on the transferring member. The color toner image is then transferred to a recording sheet. The recording sheet is then heated and pressed to fix the color toner image to the surface of the recording sheet.
The toner layer in the deep color parts of the color toner image has a large thickness because a plurality of color toners are superposed. In contrast, the toner layer in the light color parts of the color toner image has a small thickness. Especially, the white background parts have no toner layer.
Consequently, the height of the top layer of the image differs depending on color density. The deep color parts have a gloss as if painted in oil color. In contrast, the light color parts, especially the white background parts hardly have a gloss. Therefore, the gloss of the entire image area is uneven.
To solve the above-mentioned problem, Japanese Patent Laid-Open No. 2000-147863 (see FIG. 10) proposes to place transparent toner in the parts where the amount of color toners is small and the thickness of toner layer is small. Since the thickness of toner layer is substantially equalized throughout the color image, the irregularity of the surface of the color image is eliminated, and therefore the color image has a uniform gloss.
The total amount of toner per unit area, that is to say, the sum of the amount of color toners containing colorant and the amount of transparent toner hardly containing colorant per unit area, is uniform throughout the image. Therefore, the uniformity of gloss throughout the image is improved.
As an example of a color image forming apparatus using such transparent toner, a so-called tandem-type image forming apparatus is known. This tandem-type image forming apparatus includes a plurality of image carriers for forming different color toner images, and is discussed in, for example, Japanese Patent Laid-Open No. 2002-214871. The plurality of image carriers are arranged tandemly along a moving direction of an intermediate transferring belt. Each image carrier is provided with a charging device, an exposing device, and a developing device, which form a toner image on the image carrier. The color toner images formed on the image carriers are sequentially transferred to and superposed on the intermediate transferring belt (first transfer). The superposed toner images are then transferred to a transferring sheet (second transfer). After a subsequent fixing process, a color image is obtained.
Methods for removing residual toner on the image carrier will now be described. Hitherto, residual toner on the image carrier has been removed mainly by bringing a cleaner into contact with the image carrier. The cleaner is, for example, a brush cleaner formed of fibrous material or a blade formed of an elastomer (e.g., polyurethane rubber). However, this method has the following problem. Since the cleaner is in contact with the image carrier, the image carrier is gradually abraded with use, and therefore the life of the image carrier is short.
To solve this problem, an electrophotographic method that does not use a cleaner (cleanerless method) is proposed.
The cleanerless method has been recently developed in order to miniaturize image forming apparatuses and reduce waste toner. The image forming apparatus using this method has no cleaners. The residual toner on the photoconductor is picked up by the developing device at the same time as development, stored in the developing device, and reused. An example of a cleanerless system using charging rollers is given in Japanese Patent Laid-Open No. 10-247036.
In an above-described tandem-type image forming apparatus, the so-called “retransfer” can occur. That is to say, a toner transferred from an image carrier to the intermediate transferring belt can then adhere to another image carrier on the downstream side in the moving direction of the intermediate transferring belt. In the case where the tandem-type image forming apparatus has cleaners, if a toner is retransferred to a downstream-side image carrier, the extraneous unwanted toner is picked up by the cleaner and therefore is not mixed into another toner stored in the developer. However, in the case where the tandem-type image forming apparatus has the cleanerless structure, of course, the apparatus has no cleaners. A retransferred toner is picked up by the developing device, and therefore color mixing (toner mixing) can occur.
When transparent toner is used in order to equalize the total amount of toner per unit area throughout an image, the transparent toner is placed mainly in the non-image part. If color toners are mixed into the transparent toner stored in the developing device, the color toners are placed in the non-image part. This is the same as the so-called “fog.” In extreme cases, this causes a defect in the image.